The USA generates about 20% of its electricity from natural gas;
699.6 billion kWh [1] were produced from gas in 2004. This
consumed 6,020,335 million ft^3 of gas [2] (roughly 6.2 quads)
of our total consumption. The net efficiency was roughly 39%;
the other 61% of the energy turned into heat. Only 31% of
this gas-fired generation was in combined heat and power plants;
the heat from the other 69% was discarded.

The 60.5 million households using natural gas for heating in 2001
used a total of 3.32 quadrillion BTU of gas for heating [3], or
roughly 55 million BTU each. The 8.5 million households heating
with fuel oil [3] used 0.58 quads, or 68 million BTU each. The
6.6 million households heating with LPG used 0.28 quads, or 42 million
BTU each. Together, these 75.6 million households used a total
of 4.18 quads of heating fuel in 2001, averaging 55 million BTU each.

Commercial buildings are similar. In 1999, commercial space used
1.76 quads of natural gas and .17 quads of fuel oil [4] for heating.
Together, the fuel used for heating residential and commercial real estate
came to 6.1 quads. This is nearly the same amount of energy as all
the natural gas used for electric generation, and most of it is consumed
during the half-year of the heating season.

Home-heating oil is almost the same as diesel fuel, and both LPG and
natural gas are high-octane motor fuels; every home and business which
has heating fuel delivered in tanks or pipes could run a generator to
provide its electricity too. The generator converts part of the
fuel's energy to electricity and the rest to heat. Creating the
heat where heat is needed is better than making it where it has to be
discarded.

Cogenerating electricity with heat creates large efficiencies and could
have eliminated the prospect of a natural gas shortage this winter... if
we had done it. How much? Here's an example:

Suppose the typical house uses 55 million BTU of gas for space heat over
the heating season (average 367,000 BTU/day over 150 days) and consumes
15 kWh/day of electricity over the same interval (2250 kWh total). If
the electricity was generated in a 39%-efficient gas-fired plant, it would
require 19.7 million BTU of gas. The total for heat and electricity
would be 74.7 million BTU of gas. If 63 million BTU of gas was burned
in a generator at 12.2% electric efficiency and 95% overall efficiency, it
would make 7.68 million BTU of electricity (2250 kWh) and 52.2 million BTU of
space heat (what a 95%-efficient furnace would deliver from 55 million
BTU of gas). This would both heat and power the house on about 15% less
fuel. This could easily make the difference between a crisis and a yawn.

12.2% is a rather low efficiency for a generator; small diesel engines can
reach 30% without undue difficulty. Suppose that the houses heated by
natural gas and LPG use generators getting 25% electric efficiency (95%
overall), and the ones heating with oil get 28%/95% out of theirs. These
houses would burn some extra fuel but generate large excesses of electricity.
This electricity could run electric vehicles (displacing motor fuel and making
it available for home-heating oil), heat pumps (cutting total fuel demand
further), or go for other purposes.

A cogenerator running at 30% electric efficiency and 95% overall efficiency
would burn about 46% more fuel than a furnace for the same amount of heat,
but it would have an electric output of 46% of its heat output. If this
electricity ran a heat pump with a typical 3:1 coefficient of performance (3
BTU of heat out for each BTU of electricity in), the total heat available
from the fuel would be 238% as much as the original demand (100% from the
cogenerator and 138% from the heat pump). The net heating efficiency
would be 155% (65% cogenerator, 30%*3 = 90% heat pump), squeezing 63% more
heat out of each unit of fuel. Again, this is enough to turn a fuel
crisis into a yawn.

It's gotten warm in Michigan this past week, but people are either looking at
their heating bills from the previous cold snap or are waiting in uneasy anticipation
thereof. The US chemical and fertilizer industries are dying because of sky-high
natural gas prices. Cogeneration could have made things a lot better. Our
current situation is entirely due to our own refusal to be ready for the future
that's coming at us - ready or not.

EP, what sort of barriers do homeowners who may want to install a cogenerating furnace face? I'll be in apartments for quite a long time, but when I do eventually own a home I would love to do stuff like this.

1. Lack of SCADA systems capable of managing large numbers of cogenerators.2. Lack of pollution laws recognizing the unique characteristics of cogenerating heat plants (e.g. NOx contributes to smog, but smog is almost never a problem during cold weather as its formation is affected strongly by heat).

On the individual scale:

1. Lack of electricity resale laws which allow full cost recovery for excess production (this goes beyond net metering).2. Lack of tariffs which prevent excessive interconnection or metering charges for the "unusual" setup.

Without the appropriate initiatives, all of these pitfalls are bound to be put in the way of domestic cogeneration. Some may be appropriate for e.g. Denver or other sites where pollution is a particular difficulty even in winter, but the majority of these things should be waived where the problems are small or can be worked around.

Okay, I'll bite. How much of an up-front expense are we talking about for such a system? The homeowners who are being hosed by an extra $200/month in heating costs aren't in a good position to invest in $10k of electric generation cogen and grid hookups.

IIRC, Climate Energy's (Honda-based) system is supposed to cost about $8000; this is about $4750 more than I just paid for a fairly modern condensing furnace, installed. It would take a while to recoup that much by saving a fraction of $200/month.

I don't believe that the cost needs to be that high, and the useful output could be greater with a different design. That would offer a much faster payback. How fast, I can't say; I haven't been able to get figures for the various components which would allow me to pin down a system cost. On the other hand, when you can buy a 4.4 kW engine from India for $1000 it's obvious that there's a lot of room to cut prices.

1. A large fraction of the nat gas used for electricity generation is used in peaking units, particularly so to meet air conditioning loads in summer. It's also the peaking units which have the lowest efficiency.

In winter, co-generation systems would have to compete with base load nat gas, ie likely the most efficient combined cycle plants (and also in many cases co-generation systems in industry).

2. Heating loads vary a lot, even where co-generation systems are installed for households (which is virtually only where they are very, very heavily subsidised, eg in Germany), the generator will usually be sized for only part of the maximum heat load, with an extra furnace in place. The reason for this is the high capital cost of a system that would only run very occasionally.

3. I gather that emissions for engines are higher than for furnaces, and I've also noticed that the co-generation systems I've seen do not include a condensing option.

Without the condensing option and considering the low electrical efficiency, however, the nat gas saving becomes negligible.

----------------------(by the way, I've decided to start a blog on blogspot as well)----------------------

On climate change

I think the discussion isn't so much about sceptics questioning the "consensus", but rather one about the relative emphasis on adaptation over mitigation.

My own view is that mitigation should largely be restricted to research, and that at this stage carbon dioxide emissions should not be penalised. I think the appropriate level for carbon taxes is zero.

The IPCC does not take a position on this question, ie the relative contributions of adaptation and mitigation.

And I might note that in practise, climate change policy across the globe largely favours adaptation. Europe and Canada may have signed onto Kyoto, but they've refused to put expensive and unpopular measures in place to meet their Kyoto obligations.

The current world average carbon tax is pretty close to what I consider ideal, ie in most places it's zero, and in Europe it's low (the system in place isn't a tax I believe with the exception of Norway - in the UK there's a climate change levy, but it also applies to nuclear power and is more like an energy tax - and in the rest of Europe there's now a tradable emissions certificate system, but the price of those certificates is not equivalent to a tax, because large emitters have been given those for free, and only have to buy a fraction of their needs on the market, if they choose this option rather than reducing their own emissions).

"In winter, co-generation systems would have to compete with base load nat gas, ie likely the most efficient combined cycle plants (and also in many cases co-generation systems in industry)."

Base-load natural gas is doomed in N. America. Most of it never should have been built.

Even using Climate Energy's figures (20% electric efficiency, 85% total efficiency) it is better to burn gas in the cogenerator than a furnace. If the cogenerator production is offset by the demand of a heat pump with CoP of 3.0, then each BTU of gas to the cogenerator makes .65 BTU of heat on the spot and another 0.60 BTU of heat from the heat pump. The total useful heat is 1.25 BTU per BTU of gas, roughly 1/3 greater than the best furnace available.

This only gets better as the generator efficiency goes up.

"the generator will usually be sized for only part of the maximum heat load.... The reason for this is the high capital cost of a system that would only run very occasionally."

The cost of the system is determined mostly by parts count, and only weakly related to size. The difference between a 1 kW system and a 4 kW system is small, because they take almost the same number of parts and machining steps. Cycling the system on and off doesn't make it cost more, and eliminating a backup furnace might make it cost considerably less.

Pollution is an issue, but it may be over-regulated. NOx is the big difficulty for small generators because the catalytic reduction systems are expensive and difficult to maintain. But the reason for controlling NOx is to reduce photochemical smog (ozone). The production of smog also requires hydrocarbons and sunlight, and only proceeds rapidly with heat. When temperatures hover around freezing, would there be significant ozone even from uncontrolled gas-fired generators? If not, there is no reason to regulate them the same as generators intended to run during A/C peaks.

"... there's now a tradable emissions certificate system, but the price of those certificates is not equivalent to a tax, because large emitters have been given those for free..."

Meaning that would-be less-emitting competitors have to buy permits from the incumbents just to get into the game, and international competitors don't have the overhead of compliance either. In short, the system's incentives are as perverse as the USA's CAFE regulations, which is why it works so poorly.

sums to 101.75 kWh, or a negligible saving compared to the large capital expense and the loss of flexibility (electricity production at least partially tied to heat production rather than entirely to electricity demand)

The condensing option is a must for the co-generation system to make any sense in my opinion.

Is the 60% figure based on the HHV or the LHV? If it's not based on the HHV, the comparison equates incommensurables.

And an engine built with 1930's technology yields 30% efficiency, and a more modern one might get 40% (some diesels do). If you get 40% work out plus heat and condense the exhaust to achieve 95% overall, you get 55% direct + 120% heat pump = 175%, roughly the same (I could do better using an absorption cycle heat pump running off the engine's heat, boosting by at least 27%). You also get:

I thought all figures we were discussing were on a LHV basis, things do change slightly if the 20/85 and 95% figures you were giving are on a HHV basis.

Things will change even more, if you assume 40% for the engine, as you point out.

There are also potential side benefits as you say.

However, I'd also say that the co-generation systems being sold here in Europe don't measure up to your assumptions, either the electrical efficiency is much lower (Whispergen), or the price is much higher (Dachs) and the electrical efficiency is still clearly below 30%.

I think emergency generation is a clear advantage, though I am not sure whether it's worth very much to the average consumer who gets very few power outages. Grid issues are complex, the (at least partial) dependency on heat loads may make grid management more difficult, in spite of some demand being met more locally.

Incremental deployment is an advantage. On the other hand, I've heard the argument that low production volumes are (at least partially) responsible for the currently astronomical prices of the available single family home sized units, so said partial deployment also seems to have some disadvantages.

As for multi-fuel capabilities, I am not sure what the advantage here exactly is. Homes can switch to woody biomass derived pellets, and large coal fired power stations can co-fire woody biomass, all at quite reasonable efficiency and with comparatively much lower capital costs.

The exhaust temperature of many condensing furnaces is in the range of "warm to the touch". These are clearly recovering much of the heat of condensation of the water, so their sub-100% efficiencies are all but certainly rated by the HHV.

I looked at Whispergen some years ago, and calculated an efficiency around 11%. But one bad implementation does not mean the idea is wrong.

Last, the cost of single-family homes is not due to incremental deployment, it's due to the fact that they are largely hand-built. So-called "mobile homes" (manufactured houses built on trailers) are much cheaper per square foot, and some "modular" buildings get some of the economies of scale by building in factories.

Lamborghinis are largely hand-built, Volkswagens are made by the hundreds of thousands per year to almost identical specs. A line of cogenerators built using automotive assembly-line techniques would have the same cost advantages of the Volkswagen; it would probably be feasible to build and assemble the components on some of the same lines.

Your English is about as good as mine, though we may be tripping over idiom.

The potential economies of scale are one of the reasons the cogenerator scheme is attractive. The auto industry is very good at making large numbers of engines in the 1.5-liter range!

Let's see, the heat of combustion of methane is 212.79 kcal/mol (measured at 25 C). Each mole yields 2 moles of water, which has a heat of vaporization of ~540 cal/gram, or 19.44 kcal per mole of methane. Burned with air with 0% excess oxygen and assuming 20% oxygen by volume, the product gas would be about 19.2% water vapor by volume. That's a partial pressure of about 19 kPa.

At 40°C, water has a vapor pressure of about 7.4 kPa. If the exhaust temperature was lowered to 40°C, about 61% of the water would condense, yielding about 12 kcal/mol of methane. If you were measuring against the LHV of methane (~193 kcal/mol), the condensing furnace would have an efficiency over 100%. Since even the best are rated less than that, I'm assuming that they are using the HHV (I've had no luck finding a definition of AFUE which specifies HHV or LHV).

Thanks for the hearty debate here E-P and Heiko. Excellent post. The potential of distributed cogeneration should really be explored and utilized where applicable.

I'm sure the costs could come down if economies of scale are realized and I imagine the pollution concerns could be avoided in at least enough areas to provide a market for distributed cogen if the appropriate legislation cleared the way. A few incentives to get things rolling might not be a bad idea as well - I know you hate subsidies E-P, but just to get things started and to drive demand so that economies of scale can be realized. I believe that some states already include cogen in their 'renewable energy' incentives (along with distributed wind, solar etc.).

Also, thought you might be interested to know that Stirling Energy Systems, who has enjoyed some recent attention due to their large contracts for solar farms using their dish-stirling solar concentrators, is also looking to market their stirling engine gen-sets for the distributed generation market.

I imagine such a gen-set could be easily coupled into a co-gen system and could take advantage of economies of scale driven by their solar concentrator sales. I also imagine that the efficiency of the stirling engine system would be pretty high. Just a thought.

Also, what is LHV and HHV and the difference between the two? Sorry for not being up on all the acronyms...

Stirling Energy Systems doesn't mention a price, darn it. It's impossible to even guesstimate the practicality of things without a pricetag.

The higher heating value (HHV) and lower heating value (LHV) are the energy yield from a fuel with and without condensation of the water produced, respectively. For e.g. charcoal there's no real difference, but for hydrogen, methane and propane the difference is substantial.

But they do mention the efficiency (LHV basis). It's 88% for their standard version, 89% for their low Nox version and 98%, if you also buy the extra heat exchanger they now offer (I haven't been able to get a price for that extra though).

The electrical efficiency is 27% and the service interval 3500 hours.

That, by the way, is an important difference between cars and combined heat and power plants like the Dachs. A car running at 100 km/h does a mere 100 h before it gets serviced (usually every 10,000 km).

It seems that getting maintenance costs down was (still is? not sure on that one) a major issue.

The Dachs is a bit too large for a family home.

My parents have 180 m2 and consume 18000 kWh of heat per year (provided by a heat pump). That happens to also be the German average for new build (100 kWh per m2).

12.5 kW thermal is really too much for that. The year's got over 8000 hours, so the Dachs could produce nearly 110,000 kWh of heat giving a utilisation of only about 16%.

Anyway, we can now redo the calculation with the numbers I've gathered (all LHV basis):

88% overall efficiency

27 kWh of electricity61 kWh of heat

Produce heat at 104% in condensing furnace

61/1.04=58.65 kWh of nat gas (LHV)

Produce electricity at 60% in combined cycle

27/0.6=45 kWh of nat gas (LHV)

Sums to 103.65 kWh of nat, giving a 3.65% saving.

With condensing option

98% efficiency

27 kWh of electricity72 kWh of heat

Now we'd need to use 69.2 kWh of nat gas to produce the 72 kWh of heat, and the numbers sum to 114 kWh.

Now figure the T&D costs and losses that the cogenerator does not have. The losses average 7% in the US, IIRC, and the T&D system is about as expensive as the powerplants. The advantage is not spectacular, but it still goes to the cogenerator. And as efficiency improves, it gets even more lopsided.

I was just reading over at The Energy Blog and came across the piece on MCFC's. 60% efficiency, and heat recovery too. They aren't yet small enough for stand-alone dwellings either, but that may come. Or maybe it will be low-temperature SOFC's.

Last, you're not playing fair to the cogenerator. So it would have an annual duty cycle of 16% at your parents' home. A heating plant needs to be sized for peak loads, not average; what's the annual duty cycle on their heat pump?

If you multiply the efficiency of the central system by .93 to allow for T&D losses, the domestic cogenerator looks substantially better. For that matter, not everyone would need to have one; if a fraction of the neighborhood had cogenerators and the rest used heat pumps, the cogenerators could supply most of the power required by the heat pumps (since they are responding to similar demands). The average transmission distance for the power could be under 100 meters.

Last, cars and stationary engines have very different operating conditions. A Lister-type engine chugging along at 650 RPM would turn 1/3 to 1/4 the revs of a car on the highway, gaseous fuels cause none of the oil contamination problems of gasoline or diesel, and the engine can always be run up to full temperature to purge the oil of moisture before it shuts down. Maintenance intervals in such engines can be many months, and 20 years between overhauls. I'm sure somebody can come up with an oil reservoir which can be swapped in two minutes, which you'd do once a year.

That's just engineering and selection of the operating regime. It's no big difficulty.

Unfortunately, turbines lose the efficiency contest at small sizes (where heat leakage and viscous drag of small passages dominate). For instance, Capstone claims 26% efficiency for their 30-kW microturbine; a 4.4 kW Lister-type diesel (single-cylinder) is rated at about 30%.

Transporting nat gas also requires energy, don't forget, and the infrastructure cost for the network may not change at all, if peak demand is in summer.

Also don't forget the tying of heat demand and electricity generation, which cuts the flexibility of the system (I've heard of problems in Denmark, where in some cold windy nights co-generator generated electricity had to be sold for nothing to the German network - the grid operator was obliged to take it, but couldn't do anything with it but sell it on at a loss).

I think there's a small efficiency gain, but I think the advantage goes to larger co-generators, furnaces, centralised power stations, better insulation ... The capital requirement for the emissions reduction is currently way too large for micro-scale CHP compared to what else could be done with the capital.

That may change, I am not convinced myself that merely churning out loads of subsidised units would be enough though, many of the components are already pretty well optimised. I don't think that the cost reductions and efficiency improvements from predictable economies of scale and incremental technological change are anywhere near enough even with millions of units being built (at a cost of tens of billions of Euros).

I phoned my father about his heat pump. He thinks it's about 10 kW thermal, while the nat gas fired back-up furnace is rated 17 kW. Half the power is night time electricity, half day time. 95% of the heat is supplied by the heat pump.

The back-up furnace is sized to provide all the necessary heat (and then some spare) on the coldest day of the year with the heat pump out of action. The furnace is cheap. My father bought the heat pump for around 30,000 Euros 25 years ago. It supplies heat at about half the cost of nat gas, but that's only a saving of about 500 Euros per year (at present nat gas prices).

---------

On the turbines. One of my colleagues thinks there's potential for vehicle applications. But (disclosure) he may be biased, as he's one of the very few people who actually worked on turbines for hybrid trucks before the plug got pulled. I wouldn't really want to say anymore on that without his agreement though.

It wasn't a stupid question, Alex. The average gas turbine is quite a bit more efficient than the average piston engine; it's also a lot bigger, and people without specific subject-matter knowledge would have reason to believe as you did. As for the crossover point, I'm not sure you can point to a specific figure; the Wartsila-Sulzer 2-stroke marine diesels can beat 50%, but they are very big, burn oil rather than coal or gas and come no bigger than 80 megawatts. As far as I know, the GE simple-cycle turbines get about 40% at the 300 megawatt size and 38.6% at 30 megawatts. There are general trends but individual design features weigh heavily.

Heiko: You don't have to tie generation to heat demand, you just have the option of serving demand that way. If you have a rough balance of cogenerated supply to heat-pump demand, you could feed cold-weather needs from the cogenerators, from non-local electric generation (such as wind farms, with the cogenerator-served buildings falling back to resistance heat) or any combination. It gives you maximum flexibility, and it would allow the system to absorb large amounts of renewable energy from wind farms and wave generators without also relying on their presence.

I know you don't have to, but if you don't run the co-gen sets as co-gen sets, but as furnaces, or worse use resistance heat from wind, you've got an extremely expensive piece of kit that needs to be amortised over as many operating hours as feasible sitting about doing nothing (I don't think you really pick up on the main issue I have with heat pumps and micro CHP, namely the large capital cost and the fact that there are better uses for that capital in terms of emissions reductions).

On wind, we are in perfect agreement there. With wind blowing more in winter than in summer (in Northern Europe at any rate), one way to deal with its variability is to use some of it for heating (resistance or heat pumps).

Also, a good paper on how to deal with the variability of wind through long distance electricity transmision:

"you've got an extremely expensive piece of kit that needs to be amortised over as many operating hours as feasible..."

Let's address those assumptions one at a time:1. A cogenerator is smaller and simpler than a vehicle's drivetrain, which auto makers build for $5000 or less. I've seen prices for 1.4-liter Lister clones as high as $1800 and as low as under $1000. Units like this built in volume would be anything but "extremely expensive".

I just paid more than $3000 for a new furnace; if I had 2 million annual sales to justify development of switched-reluctance alternators and other ways to apply smarts to the construction of dumb, reliable cogenerators, I'll bet that the $3000 target is reachable.

2. Once installed, the cogenerator is a sunk cost. It should be operated when it is cheaper than other energy supplies, and left idle the rest of the time. You were the one who said that maintenance was required every few hundred hours. Is a thousand hours a year a problem?

"(I don't think you really pick up on the main issue I have with heat pumps and micro CHP, namely the large capital cost and the fact that there are better uses for that capital in terms of emissions reductions)."

Given your position that emissions are unimportant, I wonder if you've given that enough thought.

The difficulty I have with the all-heat pump scheme is that it requires the same inefficiency of peaking generation that's seen in hot areas during the summer (the generation peak in Minnesota is during the winter). If you are forced to meet the marginal demand with simple-cycle gas turbines at 40% efficiency tops, you get a best-case AFUE of 120% assuming 3:1 heat pumps; add in 15% line losses during max-load conditions and you're down to 102%.

Off-the-shelf engine hardware looks like it can already yield 155% efficiency from the cogenerator/heat pump scheme (even under max-load) and future developments could increase that to over 200%. The system also delivers emergency power in the bargain. How can you get that any other way?

"if you use resistance heat from wind..."

... you1. offset fuel use and cogenerator operating hours using extremely cheap resistance heat hardware.2. provide a dump load for wind power that might otherwise go unused.3. provide a market for excess wind power no cheaper than the displaced cost of cogenerator fuel and maintenance.4. provide a set of alternate generators which can be scheduled from zero to full power on a time-scale of seconds, making it much easier to handle any variability of the wind resource.

I propose using both because each one addresses deficiencies of the other; they are synergestic.

1. I know that you can pick up a 1 kW petrol generator for around a hundred pounds, but I also know that the 5 kW Dachs sells for 25,000 Euros. I believe the main issues are noise, durability, maintenance and emissions (other than CO2) and I am personally skeptical about large capital cost reductions, and the main reason for my skepticism is that there is good subsidies available in Europe and people just don't seem to be able to get the cost down much even with all that help.

2. Yes, once bought the cost is sunk. That applies to co-gen sets as much as it does to nukes or wind turbines.

On carbon dioxide emissions and capital costs. We've got two separate questions here. Firstly, what should be the carbon price in Dollars per tonne and secondly what is the emissions reduction cost of a particular option.

I accept that there is a willingness to spend money to reduce carbon dioxide emissions now, while I only want research and development and see value in preparing for possible future reductions in carbon dioxide emissions. Given this, I think it's worthwhile for me to contribute to the debate on what options are the most effective.

On heat pumps and efficiency. Look, heat is cheap and there's all sorts of other ways to deal with the issue, not least improved insulation and counter-current heat exchangers (heating fresh, cold incoming air with stale, warm air that it is being exchanged with).

Old housing in Germany consumes something like 300 kWh per m2, new build is down to 100 kWh per m2, and low energy eco-housing gets down to 40 kWh per m2.

With that little a heat load, why not use a small pellet stove combined with plain electrical heating for example?

On wind, as said we pretty much agree here. If wind is to be used for 70%+ of electricity generation (that's not my preferred option, but that's entirely a separate issue), there are huge advantages to using some of the wind generated electricity for resistance heating. It would mean that there'd be no waste, even when heavy winds would otherwise imply a 100% excess.